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HARDWARE AND SOFTWARE SELECTION FOR EXPER1MENTAL MODAL ANALYSIS

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Abstract. CommerciaJly availab/a experimantal moda! analysis sy storn s typka/fy contist o fan FFT anaiyzer, a moda! anatysis procassor. a graphks tar mina!, and a storaga dayica. Digital piottars, chan net ss/ectors, printan, hard copy unita, and othar accassorias can ba intarfacad, and tha operatbn of tha oyarail system can ba coordinatad through a host Computer to anhanca rts capabi/ity. Tha seiaction of hardware for a partfcular appiication shou/d address apacific objecthras as wail as hardware capabilities. Software se/ection is aquaily importem Proper seiaction is diffkuit unłass tha undariying thaory is undarstood. In particufar, datarmination of transfer functions via FFT anatysis; extraction of natura! freęuencies; moda/ damping ratios and moda shapes from transfer function data; and tha construction o f mass, stiffnass, and damping matrkat from moda/ data śioutd ba considarad. Tha papar addresses thass issuat Four commarcia/fy avai/ab/a moda/ anatysis Systems ara considarad in a comparatłva eealuatfon.

In today's industry dynamie testing is being exten-sively incorporated into the design development, quality control, and qualification of Products. A wide spectrum of device$ and components ranging from large missiles. reactor coolant loops, and auto-mobiles to delicate sensors, transducers, and micro-processor moduł es is tested in this manner. The primary objective of testing is to determine the dynamie behavior of the test object. This informa-tłon can be used for fault diagnosis, design improve-ment, and evaluation of the operationał capability of a system.

Dynamie tests typically involve applying a measured forcing excitation along a natural direction (degree of freedom) of the system and measuring the system response 8t that location as well as at other critical degrees of freedom. The wide choice of forcing excitations available •• impulse. sine sweep, sine dwell, sine beats, narrow-band random, and broadband random - depend on the 90urce of excitation. An instrumented hammer is used in impact (bump) testing. A portable shaker, typically electromagnetic, or a large shaker table, typically hydraulic, can be employed to provide the forcing excitation in single frequency, multi*frequency. or random testing (1),

Analyzing the measured response signals (time histories) allows determination of a variety of fre-quency domain and time domain Information. In experimental modal analysis this information pri-marily consists of natural frequencies, modal damping, and modę shapes. If an adequate number of degrees of freedom is monitored during testing, it is possible to determine a complete dynamie model for the system in the time domain; specifically, mass matrix, stiffness matrix, and damping matrix can be extracted from test data.

This paper discusses several basie issues that should be considered in selecting a commercial stand-alone modal analysis system, from the point of view of hardware and software capabilities. Background theory pertaining to today's modal analysis software is introduced at the outset because it is important in understanding the functional operation of modal analysis systems.

ELEMENTARY THEORY OF EXPERIMENTAL MODAL ANALYSIS

The primary objective of experimental modal analysis is to extract natural frequencies, modal damping, modę shapes. mass matrix, stiffness matrix. and damping matrix of a dynamie system from a set of measured responses at crucial degrees of freedom (d.o.f.) for a known excitation applied at one of them at a time.

•Department of Machenlcat Enginaarlng, Oamaęta Mellon Un/renłty, Schantay Park. Pfttsburgh. PA 16213